Author Topic: A broken HP 33120A (Read 7813 times)

vaualbus · « **on:** May 01, 2014, 07:03:28 pm »

I've recently bought a broken HP/Agilent 33120A with the stable reference in function generator for around 200 bucks.
Yesterday it arrive home, I turn on and for like 5 minutes it turn on and fail self test.
Than the fuse blowed.
I open the instruemnt and I've found a shorted diode CR704 a broken diode CR803 and four tatalium caps burned (one have also demage the PCB unfortunally).
I post some photo of the main board.
What I think it happens is that tese caps burn and take down the two diodes. (Strangly one of the two diode is cracked but still test good meanwhile the other is a short circuit).

Image 1: https://www.dropbox.com/s/gqvwzb979m0lru9/1.jpg

Image 2: https://www.dropbox.com/s/gzpujw8xk2i8vby/3.jpg

Image 3: https://www.dropbox.com/s/l8j6m6ntb0uubah/6.jpg

Best regards, Alberto.

vaualbus · « **Reply #1 on:** May 01, 2014, 08:40:08 pm »

I have some problem with dropbox either on my machine or on the server. For now only the first image is visible.

HighVoltage · « **Reply #2 on:** May 01, 2014, 09:31:24 pm »

Can you tell us what the error message was during startup or selftest
I also have one 33120A that is dead but the problem is not clearly to be identified

vaualbus · « **Reply #3 on:** May 02, 2014, 05:23:19 am »

I can't now the instrmwnt doesn't turn on. I need to replace the csps and diodes.

pgross · « **Reply #4 on:** May 02, 2014, 11:32:54 am »

Is the fried tantalums in your 33120A - 20V rated types?

I know that a couple of 20V rated tantalums in the 33120A are mounted directly across the +/- 15V rails.

It almost looks like the same issue with the 34401A multimeter, where some have discovered 20V types
tanatalums have fried and damaged the pcb.

If the “normal practice” of min. 50% voltage derating should be applied, these tantalums should be
retrofitted to 35V types.

Hope that you’re able to clean / repair the pcb, and get it functional again.

Best Regards

Peter

vaualbus · « **Reply #5 on:** May 02, 2014, 11:54:17 am »

Ya they are 2.2uF 20V caps. I wish that the cap that have burned the pcb havem't demega the internal layers.

TiN · « **Reply #6 on:** May 02, 2014, 12:09:15 pm »

I have donor board

vaualbus · « **Reply #7 on:** May 02, 2014, 02:40:28 pm »

Next week I'll go and buy the component (on of the two diode, on rs components it will be avaible only in december so I need to buy that on ebay)
and I will update for the results.
Best regards, Alberto.

vaualbus · « **Reply #8 on:** May 04, 2014, 10:42:17 am »

>Any body kno why tantalium capicitor burn when they get very old?

nctnico · « **Reply #9 on:** May 04, 2014, 02:11:26 pm »

Tantalum caps should be used in places where the current is limited. Tantalum caps basically are made of thermite. Too much current= iginite. Reverse voltage = ignite. If the circuit doesn't depend on the ESR of tantalum caps then it is better to replace them with low ESR electrolytic or ceramic capacitors (X5R or X7R dielectric).

vaualbus · « **Reply #10 on:** May 04, 2014, 02:28:52 pm »

so you suggest to use normal eletroliti caps in the 33120A? All this caps are bypass capicitor on the -18V and 18v rail?
If it is possible it even better for so I haven't to buy the caps.

vaualbus · « **Reply #11 on:** June 04, 2014, 04:10:30 pm »

I've some update for you guys:
Fist I replaced all the caps and diode and now the display not work (The fronpanel fall from my desk so the vacum display could be broken but I don't see any crack), but the instrument still not produce any output.
The frontpanel a part from the display it work, I can move the encoder an hear the click, use buttons and ecc.
Probably I find some problem. The negative +15V read -20 meanwhile the positive +15V read only 9V.
I don't kno why I read this voltages?
PS Somebody have some frontpanel or vacum display to sell to me?

vaualbus · « **Reply #12 on:** June 04, 2014, 05:43:48 pm »

I was wrong aboat the volatages I mesearud them with the wrong GND. Both 15 an -15 are right.

vaualbus · « **Reply #13 on:** June 04, 2014, 08:02:27 pm »

Also how I can meseuare the output of the dac?
It seem a stupid question but it isn't the dac have a differential current output so I can't meseare that with a scope.
Any idea?

TiN · « **Reply #14 on:** June 04, 2014, 09:34:37 pm »

Check getter on VFD. If there is white residue around getter, it means oxygen got in and vacuum is gone now.
If its metallic shiny, your display likely be okay.

vaualbus · « **Reply #15 on:** June 04, 2014, 11:20:54 pm »

ya is white as I suspect because when I remove the vacum displat to check voltage on the front panel I saw two cracks.
Now I need to find a new display vacum tube. It seem hard to find.

vaualbus · « **Reply #16 on:** June 05, 2014, 10:53:08 am »

IU have discovered something very very strange.
So If I measeaure the +/- 15V of the output amplifier using the agnd as ground I meseaure -20V an +9
but If I meseard them using the gnd as ground I meseare +15 and -15.
The strange think is that all the bypass cap are grounded using the agnd.
What is going on?
What I should use as ground agnd, or gnd?
(The worst think could happen is that near the caps that burn and that have the demage the pcb, the two ground plane have shorted).
Anybody can meseuare the resistence between GND and AGND?

vaualbus · « **Reply #17 on:** June 08, 2014, 06:37:24 pm »

So i find somethingh strange; apparently the two ground are not isolated, there about 800ohm of resistence between.
If I meseaure the voltage across the grounds it reads, a magic number, -5V. A little bit of math with the previous measaruments explain the solution.
-15V + -5V = -20V, 15V + -5V = 9V. So the volatages I read was right the only problem is that the ground is not a zero volt.
I think this is not normal, so looking for a explnation the only component in the entire instruments that are betweeen the grounds, is the semi custom DAC.
(Fortunally it seem to be avaible in ebay).
I'm right? It seem a good explanation to me.
Best regards, ALberto

vaualbus · « **Reply #18 on:** March 16, 2024, 10:16:32 pm »

10 years later and I still am perplexed on this! I still own the instrument and never repair it. I burned like 3 or 4 DACs. I still thinking that the grounds should be the same but again have no way to check it! (I now own way more interesting stuff anyway

)
Inherently enough in schematics I see that the grounds are connected at JM1006 but that part seems no where to be found in the PCB layout. What I think is that the part is the big middle mounting hole where the plastic insert is mounted.

HighVoltage · « **Reply #19 on:** March 17, 2024, 10:24:55 am »

Wow, it has been 10 years, time flies fast, crazy!

What if you connect the grounds to bring them all to the same zero level?

vaualbus · « **Reply #20 on:** March 19, 2024, 04:13:30 pm »

Indeed and after I have fixed all the broken pads with some "surgery", I still have the output amplifier not working

I would say, nothing really changed

Basically I see that I have a signal driving Q709 but no signal on the base of Q710

I am worried that the DAC is generate a too low voltage, the SM said it should generate a 800 to 1Vpp signal but when I probe DACOUT+,DACOUT- I see only a 300mVpp signal. Now the DAC is generating an output current so it can be that I am not measuring the voltage using the correct ground.


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Author Topic: A broken HP 33120A (Read 7813 times)

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